The present invention relates generally to a method, system and device for minimizing storage and processing of ionospheric grid point corrections transmitted from WAAS satellites and, more particularly, to such a method, system, and device that utilizes GPS location data and WAAS ionospheric grid point correction data to determine an optimal number of elements in a correction array utilizing a table-driven method for minimizing the number of ionospheric grid point corrections collected from the WAAS satellites.
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and corresponding ground stations. Each satellite continually broadcasts its location in space along with the current time from an internal clock. GPS receivers can determine their position to within a few meters by receiving and analyzing signals transmitted from the satellites. To determine its location, a GPS receiver scans for satellite signals until it has acquired signals from three or more satellites. Two-dimensional locations can be determined by analyzing signals from three satellites, and three-dimensional locations can be determined by analyzing signals from four or more satellites. A GPS receiver determines its location by determining its distance from the GPS satellites based on the received signals and then triangulating these distance measurements. Commercial GPS receivers can determine their locations to an accuracy of 10 meters or less with 95 percent reliability.
Although the current GPS has been successful, it has several shortcomings. For example, GPS satellite signals are subject to errors caused by ionospheric disturbances and satellite orbit discrepancies. Ionospheric and tropospheric refraction can slow satellite signals and cause carriers and codes to diverge. Because ionospheric disturbances vary greatly from location to location, these errors are difficult to correct with civilian-type GPS receivers.
To account for these errors, the FM developed a Wide Area Augmentation System (WAAS) to improve the accuracy, availability and integrity of the GPS. The WAAS is based on a network of approximately 25 wide area ground reference stations (WRSs) that are linked to cover a service area including the entire U.S. and some areas of Canada and Mexico. Each of the WRSs has been precisely surveyed so that its exact location is known. Signals from GPS satellites are received and analyzed by the WRSs to determine errors in the signals, including errors caused by the ionospheric disturbances described above. Each WRS in the network relays its data to a wide area master station (WMS) where correction information is computed. The WMS calculates correction messages for each GPS satellite based on correction algorithms and assesses the overall integrity of the system. The correction messages are then uplinked to a pair of Geostationary Communication Satellites (GEOs) via a ground uplink system (GUS). The GEOs then broadcast the messages on the same frequency as GPS (L1, 1575.42 MHz) to GPS receivers within the coverage area of the WAAS. GPS receivers may then utilize the WAAS correction data to correct for GPS satellite signal errors caused by ionospheric disturbances and other inaccuracies. The communications satellites also act as additional navigation satellites for the GPS receivers, thus, providing additional navigation signals for position determination.
One type of information that is included in the correction messages from the GEOs is ionospheric correction data. Ionospheric corrections are broadcast for selected ionospheric grid points generally spaced at 5 degree intervals in both latitude and longitude directions. One approach is to store the correction points in a two dimensional array containing a total of 2,592 elements [(360 degrees longitude divided by 5 degrees) times (180 degrees latitude divided by 5 degrees)]. Many GPS receivers, including, for example, GPS receivers used in avionics applications and portable GPS receivers used for recreational and sport applications have limited memory and processing power and therefore cannot quickly and efficiently store and process all 2,592 ionospheric grid point correction elements. Moreover, even for GPS receivers that have sufficient memory to store all the ionospheric grid point correction data, much of the memory required to do so is wasted because only the correction points within a certain distance of a GPS receiver""s current location are relevant to the ionospheric conditions at that location.
Accordingly, there exists a need for an improved method and system for receiving ionospheric grid point corrections from WAAS satellites. Moreover, there is a need for such a method and system that benefits from the WAAS data while utilizing a minimal amount of memory and system resources.
The present invention provides an improved method, system and device for minimizing storage and processing of ionospheric grid point correction data. In accordance with the present invention, a current position of a GPS receiver or other device is determined and a boundary is created based upon the position. Grid point correction data is then obtained only for correction points within the boundary. The collected correction points are used to correct the position initially determined by the GPS receiver.
In another aspect of the invention, a method in a GPS receiver for minimizing storage and processing of ionospheric grid point correction data is provided. According to the method, a current position of a GPS receiver is determined and a boundary is created based on the position. One or more bands and one or more blocks of grid point correction data are analyzed to determine if they are within the boundary. Then, a set of correction points is obtained that are derived from the grid point correction data from correction points within the one or more blocks and the one or more bands surrounding the current position.
In a further aspect of the present invention, a computer-readable medium is provided having stored thereon a data structure for storing ionospheric grid boundaries. The data structure includes a first data field containing data representing a northernmost latitude and a second data field containing data representing a southernmost latitude. The data structure further includes a third data field containing data representing a number of latitudes in the boundary derived from the northernmost latitude and southernmost latitude. The structure also includes a table containing an entry for each of the boundary latitudes that contains a grid point latitude, an easternmost longitude, a westernmost longitude, a grid point correction array pointer and a number of grid point correction array entries. Finally, the data structure includes a fourth data field containing data representing a total number of grid point correction entries for all latitudes within the boundary.
In still another aspect of the present invention, a computer-readable medium is provided having stored thereon an ionospheric grid point bands data structure. The data structure includes multiple boundary indicator data fields for each grid point band containing data representing an indication that a grid point message within each of the grid point bands should be processed based upon a computed grid boundary. Further, a computer-readable medium is also provided having stored thereon a data structure for storing ionospheric grid point blocks. This data structure includes a grid point block table containing an entry for each of a plurality of possible ionospheric grid point bands. Each band has a plurality of possible grid point blocks and each block within each band contains an indication to process a grid point message based upon a computed grid point boundary.
In yet another aspect of the invention, a computer-readable medium is provided having stored thereon a data structure for storing ionospheric grid point corrections received from a correction message. This data structure includes a first data field containing data representing a grid point longitude and a second data field containing data representing a vertical delay estimate. The structure also has a third data field containing data representing an ionospheric vertical error indicator and a fourth data field containing data representing a reception time of the correction message. The combination of the data fields establish a correction message for a grid point.
Finally, the present invention also provides computer-readable medium having computer-executable modules. The modules include a module for creating a boundary based upon a current position of a user. The boundary is established to determine the grid point correction data to be collected. The modules also include a component for processing the grid point collection data from the WAAS messages.
These and other important aspects of the present invention are described more fully in the detailed description below.